Evaporator Protection

ABSTRACT

A heat exchange device and a method of compensating for increases in pressure in tubes within the heat exchange device is provided. The heat exchange devices includes a plurality of the tubes for carrying a first medium and a compressible feature insertable into at least one of the tubes. Expansion of the medium causes said compressible feature to compress. The compressible feature can be inert resilient material.

The present invention relates to a protector for protecting components of an evaporator from damage due to freezing and in particular for protecting tubes in an evaporator from damage caused by water freezing to form ice.

Prior art cooling and/or heating systems, such as air-cooled chillers for example, may operate in situations where the ambient temperature is below the freezing point of the cooled medium in the system. In such a situation, if the cooled medium is water and if the ambient temperature is below the freezing point of water (0° C.), then it is necessary to prevent the water from freezing inside the evaporator unit of the chiller. This is particularly important when the evaporator unit is not operating, such as when the chiller is not required to provide cooling or when a fault occurs in the chiller. When a chiller stops operating due to an unexpected event or emergency, such as a power shortage or a safety cut-out, and when the ambient temperature is low, then just after the compressor of the system stops, the condenser cools down very quickly to reach the ambient temperature level. Consequently the refrigerant pressure inside the condenser also drops quickly to reach a saturated pressure corresponding to the ambient air temperature. The refrigerant, which just prior to the chiller stopping is located in the evaporator, is subject to a drastic reduction in pressure and therefore the refrigerant will boil at a temperature level corresponding with the ambient air temperature whilst migrating to the coldest chiller system point, i.e. the condenser. Before all the refrigerant in the evaporator evaporates, a significant amount of cooling is provided to the water in the evaporator and, particularly if that water is not circulating (for example if the power shortage or safety cut-out stops the entire chiller and/or the water pumps etc.), it will rapidly freeze, forming ice.

There are typically two types of evaporator used in chillers; direct expansion evaporators in which refrigerant evaporates inside a plurality of tubes and water circulates around the outside of the tubes, and flooded evaporators in which water circulates inside the tubes and refrigerant surrounds the tubes and boils outside them. The above described situation where water freezes inside a chiller is particularly detrimental to the flooded type of evaporator. As the water freezes to ice, the volume occupied by the ice increases by approximately 10% compared with the volume occupied by the water. This volume increase significantly increases the pressure within the tubes, resulting in high stresses in the tube body, which typically causes the tubes to split, thereby allowing refrigerant and water to mix and thus damaging the chiller.

Prior art solutions to overcome the above problem have been suggested. For example, one common solution is to provide dedicated heaters, such as electrical heaters, which deliver heat to the evaporator, particularly when the chiller or the evaporator is not operating. The heaters increase the evaporator temperature and thus increase the temperature of the water (or whatever coolant is employed) in the evaporator to a temperature above its freezing point. Another known solution to the above problem is to prevent refrigerant migrating from the evaporator to the condenser when the unit is not operating. This can be achieved by providing dedicated valves in the connection between the evaporator and the condenser.

However, the above solutions are expensive and complicated to install, operate and maintain. Furthermore, the latter solution in which valves are provided in the connecting pipes produces a refrigerant pressure drop during normal operation, thereby reducing the efficiency of the chiller.

Therefore it is desirable to provide protection for evaporators, particularly although not exclusively flooded type evaporators, for conditions when the cooled medium in the evaporator at least partially freezes, and that is cost effective, easy to install and does not unduly affect the evaporator efficiency or performance.

It is an object of the present invention to provide a device and a method for protecting an evaporator against freeze damage

In accordance with a first aspect of the present invention, there is provided a heat exchange device comprising a plurality of tubes for carrying a first medium and compressible means inserted into at least one of the tubes, wherein expansion of said medium causes said compressible means to compress.

Therefore there is provided compressible means in a heat exchange device, such as an evaporator for a chiller, which compensates for the change in volume that occurs when a such as water expands during normal operation of the heat exchange device or when the device partly or wholly ceases to operate. This is particularly advantageous in a chiller having an evaporator in which tubes carrying water are immersed in a bath of refrigerant. As discussed above, when the ambient temperature of the chiller is around freezing point (0° C.) or lower, the water in the tubes can freeze, which causes stresses in the tubes and which may then spilt or crack. In accordance with the present invention, however, the compressible material compresses in response to expansion of the medium as it freezes and thus the stresses occurring in the tubes due to expansion of the medium are minimised or eliminated. Therefore it is unnecessary to provide any costly and/or complicated additional mechanisms in the heat exchange device of the present invention to prevent freezing of the medium or to prevent migration of refrigerant in the heat exchange device, since the present invention as claimed overcomes the problems that these mechanisms seek to solve.

Preferably the compressible means comprises inert resilient material. Preferably the means comprises closed cell foam, for example rubber. These materials are particularly advantageous when the medium comprises water since they are generally non-absorbent, thus preventing water being contained within the means which could otherwise interfere with the compressibility of the means as it freezes and they are capable of accepting a volume change that is comparable with the volume change that occurs as water freezes to ice. For example, the compressible means may be capable of compressing by substantially the same volume as the increase in volume that occurs as water freezes. Alternatively, the compressible means may be capable of compressing either by less than or by more than the same volume as the increase in volume that occurs as water freezes. If the compressible means compresses less than the volume increase caused by the water freezing, then it must be capable of being compressed by at least a sufficient volume such that the tubes are not damaged by expansion of the medium (i.e. the tubes may be capable of absorbing a degree of stress without being damaged). In the embodiment having a medium comprising water, since typically frozen water has a volume of about 10% greater than unfrozen water, it is desirable that the compressible means is capable of at least 10% compression under such conditions as would be encountered in the tubes i.e. by at least 10% under a pressure less than or equal to the pressure generated by water freezing when confined in a tube.

The compressible means may reside in only a part or parts of the tubes. For example, the compressible means may comprise a single means at a single location in the tube or a plurality of means regularly or irregularly spaced along the length of the tube. However, in a preferred embodiment of the heat exchange device of the present invention, the compressible means is substantially the same length as the length of the tube. This arrangement is advantageous because the compressible means can absorb pressure along the entire length of the tube and the pressure will be applied evenly across the length of the compressible means, thus allowing the compressible means to be efficiently compressed. Preferably a first end of the compressible means is attached at a first end of the tube and a second end of the compressible means is attached at a second end of the tube. More preferably, the compressible means is maintained in a position such that it is substantially coaxially aligned with the tube about a central elongate axis. This provides compressible means that is centrally aligned in the tube such that the cooled medium can flow around the entire outer surface of the compressible means. Not only does this minimise the effect of the presence of the compressible means on the water flow, but also provides a maximum surface area for compression of the compressible means, thereby providing the most efficient compensation for pressure in the tube for a given compressible means.

In a particularly preferred embodiment of the present invention, the tubes of the heat exchange device are substantially cylindrical, the compressible means are substantially cylindrical and each tube and its associated compressible means are substantially coaxially aligned. This arrangement provides a heat exchange device with good flow of the medium through the tubes as well as good compression compensation provided by the uniformly shaped compressible means.

The compressible means can have any desired shape and/or size that is suitable for the purpose, and the size and shape may depend, for example on the tube dimensions and shape and also on the medium flowing through the tube. Preferably the compressible means has a generally circular cross-section. The shape of the compressible means can contribute to increasing the water side turbulences thereby resulting in am improved overall heat transfer coefficient. Typically the material of the compressible means is distributed throughout the cross-section of the means, but in some embodiments, the compressible means may be at least partially or wholly hollow.

In a preferred embodiment, the tubes of the device each have an internal diameter and the compressible means each have a cross-sectional diameter (external diameter) when uncompressed of about 10 to 25% of the tube internal diameter. Having compressible means within this range of dimensions is particularly preferred because means with a larger diameter may affect the waterside pressure drop of the heat exchange device (due to reduction of the available area for the water to circulate and thus an increase in velocity of the water flowing within the tubes).

In an alternative embodiment of the present invention, each of the tubes of the heat exchange device and each of the compressible means is elongate and has a substantially oval cross-section. The tube and its associated compressible means are substantially coaxially aligned. Being oval in shape, the tube has a maximum internal diameter and a minimum internal diameter and the compressible means has a maximum outer or external diameter and a minimum outer diameter when uncompressed. Preferably, the compressible means has a maximum outer diameter of about 10 to 25% of the tube maximum internal diameter and a minimum outer diameter of about 10 to 25% of the tube minimum internal diameter. As discussed above with regard to the previous embodiment, this arrangement is advantageous in that it minimises the effect of the presence of the compressible means and thus minimises the waterside pressure effect.

The remaining components of the heat exchange device may comprise any conventional components suitable for use with a heat exchanger. Preferably the heat exchange device further comprises an inlet operably connected to a first end of each tube and an outlet operably connected to a second end of each tube, said inlet for supplying said medium to the tubes and to the outlet. Thus water, for example, can be provided to the system via the inlet and circulated through the tubes and around the compressible medium before leaving the system via the outlet. This is particularly advantageous because flowing water will be less susceptible to freezing than stagnant water under the same ambient temperature. Preferably the heat exchange device further comprises a shell or housing, with the plurality of tubes housed within said housing and a second medium in the housing and surrounding said tubes. Preferably the second medium comprises a refrigerant or other suitable cooling and/or heating medium. The inlet and/or the outlet may be provided as part of the housing, or by separate inlet and outlet manifolds.

The above-mentioned and other features of the various embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which:

FIG. 1 shows a cross-section of a prior art evaporator unit for a chiller having a plurality of tubes for carrying a medium;

FIG. 2 shows a cross-section of one of the tubes of FIG. 1 in various stages of freezing of the medium;

FIG. 3 shows a cross-section of an evaporator unit for a chiller in accordance with an embodiment of the present invention having a plurality of tubes for carrying a medium;

FIG. 4 shows a cross-section side view of the unit of FIG. 3; and

FIG. 5 shows a cross-section of one of the tubes of FIGS. 3 and 4 in various stages of freezing of the medium.

With reference to FIG. 1, a prior art evaporator 10 for a chiller is shown. The evaporator comprises a housing 12 which holds a liquid 30, which is typically a refrigerant as is known in the art. The evaporator also contains a plurality of tubes 20 which are immersed in the refrigerant 30 and are arranged in this embodiment such that the refrigerant completely surrounds each of the tubes 20. The tubes 20 are connected to a supply for providing a medium to be cooled 40 (see FIG. 2) through the tubes 20. The medium in an air-cooled chiller is typically water. Heat transfer between the water 40 flowing through the tubes 20 and the refrigerant 30 surrounding the tubes 20 occurs during normal operation of the evaporator 10 to cool the water 40.

However, as shown in FIGS. 2 b, c and d, when the ambient temperature of the environment in which the chiller is situated falls below the freezing point of water, and particularly if the water 40 is not circulating through the tubes 20 due, for example, to a power failure, then the water 40 begins to freeze thus forming ice 50 which begins to build in the tubes 20 from the outer edges of the tube 20 to the centre of the tube 20. As is well known, the volume of water increases as it freezes thus exerting pressure on the tube 20 as the ice 50 forms. In FIG. 2 b, the ice layer 50 is still quite thin and therefore the pressure exerted by expansion of the water/ice on the tube 20 is not sufficient to cause damage to the tube 20. however, in FIG. 2 c, the internal pressure on the tube 20 is more significant, thereby stressing the tube body 20. When a sufficient amount of the water 40 has frozen to ice 50, the internal tube pressure becomes very high causing the tube to burst or split, as shown at 22 in FIG. 2 d. Clearly this is undesirably as once the ice 50 melts, the water 40 can mix with the refrigerant 30 in the evaporator 10, causing damage to the chiller and requiring the tube 20 at least to be replaced.

FIGS. 3 and 4 show an evaporator 10 for a chiller having compressible means 60 in accordance with the present invention in each of the tubes 20 carrying water 40. The compressible means 60 is preferably an elongate resilient insert, such as a length of closed cell rubber. Each resilient insert 60 is attached at either end of the tube 20 such that it is suspended approximately centrally along the length of the tube 20, thus allowing a clear flow path for the water 40 around the outer periphery of the inset 60. Under normal operation, the water 40 enters the evaporator via a water inlet 42, flows along the tubes 20, whilst exchanging heat with the refrigerant 30 surrounding the tubes 20, and passes to the water outlet 44. When the ambient temperature falls to a temperature around or below freezing point, and particularly if the evaporator 10 ceases to function, thereby causing the refrigerant 30 surrounding the tubes 20 to boil and thus extracting energy from the tubes 20 and the water 20 they contain, the water 40 begins to freeze at the outer portions of the tubes 20, forming an annular ice section 50. As seen in FIGS. 5 a and 5 b, the initial build up of ice 50 has little effect on the tube 20 or on the resilient insert 60, since there is little pressure increase due to the small ring of ice 50. However, as a greater proportion of the water 40 freezes to ice 50, as shown in FIG. 5 c, the pressure inside the tube 20 increases thereby causing the resilient insert 60 to be compressed. Even if all of the water 40 freezes causing maximum volume/pressure increases in the tube 20, the insert 60 will compress further to compensate for the increased volume, as shown in FIG. 5 d.

Although described above in the context of an air-cooled chiller system, the principles of the present invention can be incorporated into any system in which water or other media passes through tubing or the like and which may be caused to freeze. Therefore it will be appreciated that details of the foregoing embodiments, given for purposes of illustration only, are not to be construed as limiting the scope of this invention and those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the scope of the invention as defined in the following claims. 

1. A heat exchange device comprising: a plurality of tubes for carrying a first medium; and a compressible feature insertable into at least one of the tubes, wherein expansion of the first medium causes said compressible feature to compress.
 2. The heat exchange device of claim 1, wherein the first medium comprises water.
 3. The heat exchange device of claim 1, wherein the first medium expands when an ambient temperature of the device falls below 0 degrees Centigrade.
 4. The heat exchange device of claim 1, wherein the compressible feature comprises inert resilient material.
 5. The heat exchange device of claim 1, wherein the compressible feature comprises rubber.
 6. The heat exchange device of claim 1, wherein the compressible feature comprises closed cell foam.
 7. The heat exchange device of claim 1, wherein the compressible feature is substantially the same length as a length of the tubes of the heat exchange device.
 8. The heat exchange device of claim 7, wherein a first end of the compressible feature is attached at a first end of the tubes and a second end of the compressible feature is attached at a second end of the tubes.
 9. The heat exchange device of claim 1, wherein: the tubes are substantially cylindrical, and the compressible feature is substantially cylindrical.
 10. The heat exchange device of claim 1, wherein the tubes and the compressible feature each have a substantially circular cross-section.
 11. The heat exchange device of claim 10, wherein the tubes have an internal diameter and the compressible feature has an external diameter when uncompressed of about 10 to 25% of the internal diameter of the tubes.
 12. The heat exchange device of claim 1, wherein: the tubes have a substantially oval cross-section, the compressible feature has a substantially oval cross-section, the tubes have a maximum internal diameter and a minimum internal diameter, and the compressible feature has a maximum outer diameter when uncompressed of about 10 to 25% of the maximum internal diameter of the tubes and a minimum outer diameter when uncompressed of about 10 to 25% of the minimum internal diameter of the tubes.
 13. The heat exchange device of claim 9, wherein the tubes and the compressible features are substantially coaxially aligned.
 14. The heat exchange device of claim 1, further comprising an inlet operably connected to a first end of the tubes and an outlet operably connected to a second end of the tubes, wherein the inlet is for supplying the first medium to the tubes and to the outlet.
 15. The heat exchange device of claim 1, further comprising: a housing, wherein the plurality of tubes are located within the housing; and a second medium in the housing, surrounding the tubes.
 16. The heat exchange device of claim 15, wherein the second medium comprises a refrigerant.
 17. A method of compensating for increases in pressure in a tube of a heat exchange device, the method comprising the steps of: providing a plurality of tubes; and providing a compressible features within each of the tubes, said compressible features being compressible by expansion of a medium as the medium freezes within the tubes.
 18. The heat exchange device of any preceding claim 1, wherein the compressible feature is elongate and is inserted into at least a part of at least one of the tubes.
 19. The method of claim 17, wherein the compressible feature is elongate, and further including the step of inserting the elongate compressible member into at least a part of a length of the tubes. 